FIG. 1 conceptionally illustrates in axial cross-section the construction of a wet-type multiplate clutch making use of a piston and a pressing member. The wet-type multiplate clutch 10 is equipped with an input shaft 20, an outer circumferential drum section 21 arranged integrally with the input shaft, and an output shaft 30 connected through splines with an inner circumferential hub 32 at spline grooves 31. Needless to say, the input and output directions may be reversed. With spline grooves 22 of the outer circumferential drum section 21, friction plates 50 are connected through splines, and with outer circumferential spline grooves 33 of the inner circumferential hub 32, separator plates 60 are connected through splines. These friction plates 50 and separator plates 60 are alternately arranged to make up the clutch engagement unit.
In a casing 47 for a piston mechanism arranged in a left part of the drawing, a piston 40 is accommodated such that an oil pressure chamber 41 is formed between the casing 47 and the piston 40. The piston 40 presses a pressing member 42 via a thrust bearing 71. As will be described subsequently herein, the pressing member 42 presses the clutch engagement unit, which is composed of the friction plate 50 and the separator plates 60, toward a pressing-force receiving portion 23 of the outer circumferential drum section 21 arranged in a right part of the drawing. Further, designated at numeral 48 in the drawing is a return spring interposed between the inner circumferential hub 32 and the pressing member 42.
When desired to engage the clutch, oil pressure is introduced into the oil pressure chamber 41 to press the pressing member 42 rightwards via the piston 40 and thrust bearing 71 such that the clutch engagement unit composed of the friction plates 50 and separator plates 60 is pressed toward the pressing-force receiving portion 23 of the outer circumferential drum section 21 arranged on the right side. As illustrated in the drawing, the friction plates 50 are connected through splines with the spline grooves 22 of the outer circumferential drum section 21, the separator plates 60 are connected through splines with the spline grooves 33 of the inner circumferential hub 32, and further, the inner circumferential hub 32 is connected through splines with the output shaft 30 at the spline grooves 31. Therefore, power is transmitted from the input shaft 20 to the output shaft 30.
When desired to disengage the clutch, the hydraulic pressure is drained from the oil pressure chamber 41. By the return spring 48, the pressing member 42 and piston 40 are caused to return leftwards so that the pressing force is no longer applied to the clutch engagement unit to result in disengagement of the clutch.
FIG. 2 is an axial cross-sectional view of a similar wet-type multiplate clutch. It is different from the clutch shown in FIG. 1 in that there is no discrete member corresponding to the pressing member 42, only a piston 40 is arranged, and a pressing portion 46 of the piston 40 corresponds to the pressing member 42 of the clutch depicted in FIG. 1. The remaining elements of structure are the same in both construction and reference numerals as the corresponding elements in FIG. 1. A subsequent description will, therefore, be made with respect to the pressing member 42. It should, however, be noted that the same description applies exactly to the pressing portion 46.
With reference to FIG. 3 through FIG. 6, a description will be made about the operation of the conventional apparatus. As illustrated in FIG. 3, a pressing wall 43 of the pressing member 42 and a pressing-force receiving wall 62 of the separator plate 60 arranged opposite the pressing member 42 are formed such that during the disengagement of the clutch, they radially extend at right angles relative to the central axis of the clutch (see FIG. 3). Upon engagement of the clutch, however, the pressing member 42 is flexed by a reaction force to the pressing force in a counterclockwise direction as viewed in FIGS. 1 and 3, and is brought into a state shown in FIG. 4. In this state, an outer circumferential area 44 of the pressing wall 43 of the pressing member 42 and an outer circumferential area 63 of the pressing-force receiving wall 62 of the separator plate 60 are apart from each other, while their inner circumferential areas 45 and 64 are in contact with each other. The pressing wall 43 as a pressing contact wall and the pressing-force receiving wall 62, therefore, form therebetween a wedge-shaped space, which becomes gradually greater in width toward an exterior in a radial direction, as viewed in a radially-extending, axial cross-section. As a consequence, a pressing force cannot be applied evenly from the entire area of the pressing wall 43 of the pressing member 42 to the entire area of the pressing-force receiving wall 62 of the separator plate 60. Designated at numeral 61 in the drawing is an inner circumferential spline of the separator plate 60.
FIG. 5 likewise illustrates the states of the pressing-force receiving portion 23 of the outer circumferential drum section 21 and the friction plate 50 in the conventional apparatus when the conventional apparatus is out of engagement. A pressing-force receiving wall 24 of the pressing-force receiving portion 23 and a pressing wall 53 of the friction plate 50 are formed such that, during the disengagement of the clutch, they radially extend at right angles relative to the central axis of the clutch. In the drawing, there are also depicted a spline groove 22 of the outer circumferential drum section 21, a spline 51 of the friction plate 50, and a friction lining 52 on the friction plate 50.
Upon engagement of the clutch, the pressing-force receiving portion 23 is flexed by a pressing force in a clockwise direction as viewed in FIGS. 1 and 5, and is brought into a state shown in FIG. 6. In this state, an outer circumferential area 25 of the pressing-force receiving wall 24 of the pressing-force receiving portion 23 and an outer circumferential area 54 of the pressing wall 53 of the friction plate 50 are apart from each other, while their inner circumferential areas 26 and 55 are in contact with each other. The pressing-force receiving wall 24 as a pressed contact wall and the pressing wall 53, therefore, form therebetween a wedge-shaped space, which becomes gradually greater in width toward an exterior in a radial direction, as viewed in a radially-extending, axial cross-section. As a consequence, a pressing force cannot be applied evenly from the entire area of the pressing wall 53 of the friction plate 50 to the entire area of the pressing-force receiving wall 24 of the pressing-force receiving portion 23.
FIG. 15 is a diagram in which the pressing force is plotted along an abscissa X and the position of the pressed contact wall in the radial direction is plotted along the ordinate Y. As explained above with reference to FIG. 3 through FIG. 6 as enlarged fragmentary views, the pressing member 42 and the pressing-force receiving portion 23 of the outer circumferential drum section 21 both undergo elastic deformations under the pressing force in the conventional apparatus. As mentioned above, the pressing force applied to the plate hence becomes uneven as indicated by p in FIG. 15, and the stress applied to the plate is not equal over the entire circumference of the plate. The conventional clutch, therefore, cannot avoid the inconvenience that the plates undergo partial wearing and the efficiency of power transmission is lowered.